[llvm] [VPlan] Model branch cond to enter scalar epilogue in VPlan. (PR #92651)

[Florian Hahn via llvm-commits]

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@@ -8533,9 +8504,25 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
   // modified; a basic block for the vector pre-header, followed by a region for
   // the vector loop, followed by the middle basic block. The skeleton vector
   // loop region contains a header and latch basic blocks.
+
+  // Add a check in the middle block to see if we have completed
+  // all of the iterations in the first vector loop.  Three cases:
+  // 1) If we require a scalar epilogue, there is no conditional branch as
+  //    we unconditionally branch to the scalar preheader.  Do nothing.
+  // 2) If (N - N%VF) == N, then we *don't* need to run the remainder.
+  //    Thus if tail is to be folded, we know we don't need to run the
+  //    remainder and we can use the previous value for the condition (true).
+  // 3) Otherwise, construct a runtime check.
+  bool RequiresScalarEpilogueCheck =
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fhahn wrote:

Now after updating all tests, there appears to be some changes only in the VPlan-native outer-loop vectorization path. Need to take a closer look.

https://github.com/llvm/llvm-project/pull/92651